This invention relates to the surface treatment of materials by plasma ion processing, such as plasma ion implantation, and, more particularly, to improving the efficiency and safety of such treatments.
Ion implantation is a process wherein ions are accelerated by an electrostatic potential to impact a surface of an object. The energy of the ions causes them to be imbedded beneath the surface. A sufficient concentration of the implanted ions can significantly increase the hardness of the surface.
Ion implantation traditionally accomplished by creating a beam of ions and accelerating the ions toward the surface by a sufficiently large electrostatic potential. This approach, while operable, is difficult to use on many large or irregularly shaped objects, because the beam of ions must be swept over the entire surface. Where the surface has sharp curves, bores, points, or other such features, a uniform implantation is difficult to achieve.
An alternative approach is plasma ion implantation ("PII"), which is described in U.S. Pat. No. 4,764,394. The object to be implanted is placed into a vacuum chamber. A plasma of ions is created adjacent to the surface of the object to be implanted. The object is electrostatically charged to a potential opposite to that of the ions. For example, if positively charged nitrogen ions are to be implanted, the object is negatively charged using repetitive, short-duration voltage pulses of typically about 50,000-300,000 volts (50-300 kilovolts). The nitrogen ions are attracted to the surface of the object by this accelerating potential and driven into the surface and sub-surface regions of the object. Plasma ion implantation has the advantage that the plasma of ions provides a source that is distributed around the entire surface area of the object, and uniform implantation over the entire surface area is simultaneously achieved.
One result of the impacting of the ions against the surface is the production of secondary electrons with a high energy corresponding to that of the implanted ions. The electrons are repelled from the surface and impact into the walls of the vacuum chamber in which the PII process is conducted. The secondary electrons cannot penetrate through the vacuum chamber walls. Instead, they produce X-rays that do pass through the steel vacuum chamber walls and out of the vacuum chamber walls.
These X-rays emitted from the vacuum chamber would otherwise injure persons and damage equipment in the neighborhood of the vacuum chamber, but lead or concrete shielding is usually provided to absorb the X-rays. In a PII system, the intensity of the X-ray production is dependent upon the implantation voltage and the total current of the secondary electrons incident upon the vacuum chamber walls. In a typical 100 kilovolt PII implantation of stainless steel objects in a 4 foot diameter by 8 foot long vacuum chamber, the electron current per pulse can be as high as 500 amperes. To absorb the intensity of X-rays produced at this voltage and current, lead shielding placed directly on the vacuum chamber exterior walls with a thickness of about 0.25 inches has been found to be sufficient to allow for safe, legal operation with personnel in and around the periphery of the vacuum chamber.
For implantation voltages above 100 kilovolts, the X-ray absorption of lead decreases with increasing energy of the secondary electrons. The required thickness of the shielding therefore increases with increasing voltage used in the plasma ion implantation process. For many advanced processes, the use of implantation voltages of as much as 300 kilovolts is highly advantageous, which in turn requires lead shielding about 10-20 times as thick as required for implantation voltages of 100 kilovolts. The plasma ion implantation chamber must therefore be covered with a thick lead shield, or placed into a lead room, or placed at a remote site. This requirement can be costly, hazardous, and impractical.
In addition to producing X-rays, the production of secondary electrons also contributes to an inefficiency of the PII process. For each implanted ion, there may be 1-10 secondary electrons produced, depending upon the material being implanted, the implantation voltage, and the type of ion being implanted. Each secondary electron carries energy away as an inefficiency of the process. The secondary electrons pass through the surrounding plasma without losing any substantial amount of energy to it. Most of the energy or power is absorbed by the secondary electrons, making the plasma ion implantation process inherently inefficient.
There is a need for an improvement to the plasma ion processing apparatus and process. The production of secondary electrons is a natural result of the material from which the implanted object is formed, and it is therefore not sufficient to rely upon a change in the nature of the object to avoid the problem. The present invention fulfills this need, and further provides related advantages.